Known advantages of hydraulics compared with other techniques of generation and control of energy are the maximum concentration of power compared with other solutions, the good control on movement, and the transfer of heat generated within the circuit from the unit causing the heat to the rest of the circuit and to the tank, where the heat can be easily disposed of. Other techniques of motion control (above all electromagnetic techniques) have progressed more rapidly compared with oleo-dynamics, especially because of the miniaturization and the reduction in the costs of microelectronic components, whereas the hydraulics more slowly, because these new technologies are been used prevalently for a better integration of control components (such as servo-valves) with communication networks.
Together with an inevitable image of “dirty” technology, this limited technological progress is constantly reducing the application of hydraulics from an omnipresent low-cost technique to niche application used only in situations when strictly necessary, promoting, wherever possible, more advanced, silent and clean technologies.
A greater attention for the environment requires that hydraulics reduce environmental impact, in particular, that discharge of polluting substances (such as leaks of hydraulic fluid), energy consumption and problems in working areas (such as noise) be reduced. Another improvement that would be particularly desirable for users of hydraulic components is a greater compactness of the entire system, possibly realized as “black box” to be directly installed into a piece of machinery and interfaced only with the mechanical power points and the electrical controls.
Hydraulic plants can be divided into two large categories according to the technique used for controlling the movement of actuators: pressure control or flow control.
In the past, due to the greater simplicity of component manufacture, hydraulic circuits have been always controlled by pressure, generating the maximum pressure to the pump and dropping it to the required value by means of regulating valves acting as variable bottlenecks. Due to commonality of technology, current circuits generally are constructed using the same control technique, except for extremely specialized situations. Nevertheless, this design method is very inefficient in energy terms, because it generates energy and discharges energy in excess as heat. Recently, specific control systems (called “load sensing”) have been developed for increasing energy efficiency but, in spite of that, the technique of “pressure” control keeps remains the least effective in the production of hydraulic circuits.
In contrast, the technique of flow control is much less used, and consists in the direct connection between pump and actuator, without interposition of control valves and/or redirection of flow.
Even if this technique entails a substantial increase in energy efficacy, as the pump processes just the volume necessary for the movement required by the actuator and for the pressure corresponding to the load, it has certain drawbacks. The most widespread type of actuator in the oleo-dynamic field, double effect and single stem, has no symmetry between the extension chamber and the retraction chamber; consequently, its motion involves a variation of the total quantity of fluid in the circuit that must be compensated by a collecting/discharging element, preventive a direct connection of the two doors of the actuator with the corresponding doors of the pump. This problem does not occur with symmetrical actuators (double stem of equal diameter), but the application of this type of actuators involves problems of tie and bulk, as well as an increase in costs due to a double quantity of seals on the stem, which causes this type of hydraulic circuit to be used only in niche sectors.